Transistor switches find many applications in integrated-circuit design. In analog circuits, the switch is used to implement such useful functions as sample-hold.
One of the most serious limitations of transistor switches is the charge injection effect. This is due to the charge released from the transistor channel, which is also called inversion layer beneath the gate, and injected into the connected drain and source nodes when the transistor is turned from on to off. Charge injection involves a complex process whose resulting effects depend upon a number of factors such as the dimension of the transistor, and the shape of the inversion layer in the gate area.
A number of techniques have been proposed to reduce the effect of charge injection. Some of them use additional devices attempting to cancel the injected charge, for example, using a dummy switch to absorb the charge. Some use differential switches so that the voltage variation produced by the switch pair can be cancelled as a common-mode noise. To apply such canceling techniques, a device matching is usually required, and multi-phase clocks, as well as extra bias signals, may be needed. Others try to make the operation of the circuits less sensitive to the charge injection, such as by deviating the charge, or part of it, from the critical circuit node. The objective of most of the reported technique is to reduce the impact of the charge injection after it occurs, instead of reducing the charge injection itself.
Therefore, there is a need to develop a method and circuit to eliminate charge injection, instead of dealing with the effects of the charge injection.